Objects not showing in final render
There are two possible reasons for this happening.

Reason I - Object Render Disable



You can disable specific objects from being rendered in the Outliner. It is possible you did this without knowing it.



The Camera Button to the right of the Cube Object is disabled, meaning the Cube will not be rendered. Click the camera button to turn on/off rendering.

Reason II - Render Layer Settings



It could be that you have disabled the Render Layer(s) of your object in the render layers.



In the example photo, Scene: layer 1 is active and will preview render in viewport, but since Render: layer 1 is not active, then The objects in Layer 1 will not actually render. 

More Render Layer Explanation -
The square buttons below 'Scene:' are the layers that are active and currently shown in 3D viewport. You have these correctly selected if you can see them in the 3D viewport and see them in the render preview.

The buttons below 'Layer:' are the layers that will be rendered. Select every layer that you have objects on to render. 

The buttons below 'Mask:' will cause what ever objects on this layer to cut a transparent area through your render.